Hi, I've got sort of a complex prop I need help with... the gist of it is that it needs to float. Our goal is to build this around some sort of remote control helicopter - maybe one of those fan disc things you always see at the mall. Therefore, in order to float, it's going to need to be VERY light.

I am having no trouble building the entire thing out of floral foam and craft foam, but I need something to cover it with so that it will have a smooth, paintable surface. I have thought about paperclay sanded down to a very thin layer, but I'm wondering if even that would be too heavy. Any thoughts?

Plasti-dip! It comes in a spray can and is essentially spray-on rubber. It is a sealer and a primer, so its ready to paint as soon as its done. However, it does eat styrofoam, so I would test it on a little but of the floral foam first. I know for a fact that it doesn't eat craft foam. Its also very smelly, so make sure you have an open area to spray it.

Multiple layers of either wood glue or white glue (PVA glue) mixed with a bit of water to thin it, and brushed on works. Gesso or Mod Podge work similarly.

For something a bit stronger, you can go to polyester or epoxy resin. I believe polyester resin will dissolve floral foam (floral foam, aka "Oasis foam" is made from phenolic resin, curiously, the same stuff used to make billiard balls. The More You Know!™) So you first need to paint surface in a solid layer of acrylic paint. Probably at least 2 coats. Both of these resins are commonly used in the aircraft industry. They are lightweight and strong. Even so, it won't be super durable without reinforcement, so, just try not to have any major crashes.

Super Glaze. I know for sure it'll go on insulation foam and not eat away at it. It's a 1:1 ration resin and you can buy it at home depot. It's $30 but they're HUGE bottles and will last you a long time

It's going to be really difficult to make anything that's light enough for an electric toy helicopter to keep it afloat. Even a few grams can make or break the lifting ability of those little guys.

Instead of coated foam, vacu-formed plastic might be the way to go. That way you're starting with a glossy surface, and after you form it, it's hollow and super light. If you use clear plastic, you can paint the inside so it stays high-gloss.

Additionally, it takes full-time concentration and a lot of practice to make those little copters fly even in an unmodified state. The guys at the mall kiosks make it look easy because they're doing it for their entire shift every day. To remove the skill element (or lack thereof) you might tie your prop to your costume with a bit of fishing line and put the copter on full power, so it reaches max height and stays there... like a tethered zeppelin.

To clarify, Super Glaze is an example of epoxy resin. It's used for coating things like wooden table tops.

The problem I have with Super Glaze is that it is designed to be self-leveling. That means instead of staying where you brushed it, it flows downward with gravity, trying to make a flat level surface. And it continues to do this for at least an hour. This is great for flat pieces that can be layed face up. But for more 3-dimensional shapes, it has a tendency to drip downwards, leaving naked spots towards the top of the piece, particularly on vertically oriented faces.

I prefer using the epoxy adhesive that's sold in squeezebottles. It has a thicker, more gel-like consistency so it stays where you put it. It also cures quite a bit faster. The downsize is that with a cheap brush, you can sometimes get visble brushstrokes on the finished surface. I just use a foam brush, and fix any brushstrokes with a bit of sanding after it has fully cured.

The other thing I do is use marine-grade epoxy resin from the marine supply store, and thicken it up a bit by mixing in some colloidal silica (aka "Cabosil"), but that's a messier process that takes a bit of practice. I doubt you want to get into that.

To add, I tried covering a test piece with Elmer's glue. It mostly just soaked in and the texture remained the same, but it did provide a much less porous surface, so it may be a good primer for something heavier that might eat the foam. I'll get a can of the automotive filler primer and see what happens. I'm sure it'll come in handy for another prop if it doesn't work!